Split exit pupil expander

PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATION

p-0002This application is a US National Stage Application from PCT International Application No.W02007IB000875 filed on Apr. 3, 2007 (International Publication No. W02007141606).

p-0003This application a is a continuation-in-part of and claiming priority from the PCT Application Number PCT/IB/2006/001456 filed on Jun. 2, 2006 with the International Bureau of WIPO.

TECHNICAL FIELD

p-0004The present invention relates generally to a display device and, more specifically, to a diffractive optics method that uses a plurality of diffractive elements for expanding the exit pupil of a display for viewing.

BACKGROUND ART

p-0005While it is a common practice to use a low-resolution liquid-crystal display (LCD) panel to display network information and text messages in a mobile device, it is preferred to use a high-resolution display to browse rich information content of text and images. A microdisplay-based system can provide full color pixels at 50-100 lines per mm. Such high-resolution is generally suitable for a virtual display. A virtual display typically consists of a microdisplay to provide an image and an optical arrangement for manipulating light emerging from the image in such a way that it is perceived as large as a direct view display panel. A virtual display can be monocular or binocular.

p-0006The size of the beam of light emerging from imaging optics toward the eye is called exit pupil. In a Near-to-Eye Display (NED), the exit pupil is typically less than 10 mm in diameter. Further enlarging the exit pupil makes using the virtual display significantly easier, because there is no need for the interpupillary distance (IPD) adjustment or accurate positioning of the optics with respect to the eyes.

DISCLOSURE OF THE INVENTION

p-0007According to a first aspect of the invention, an apparatus, comprising: a split substrate of optical material having a first surface and a second surface, the split substrate comprising a first part and a second part which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range; two diffractive elements disposed on the first or the second surface and configured to receive an input optical beam, wherein one of the two diffractive elements is disposed on the first part and another of the two diffractive elements is disposed on the second part, respectively, and the two diffractive elements are substantially next to each other and adjacent to the line which separates the first and the second parts; and two further diffractive elements disposed on the first or the second surface, wherein one of the two further diffractive elements is disposed on the first part and another of the two further diffractive elements is disposed on the second part, respectively, wherein at least part of the input optical beam is diffracted in the two diffractive elements to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces, and at least part of the diffracted optical beams in each of the first part and the second part is further coupled out of the split substrate by diffraction in each of the two further diffractive elements to provide substantially identical two output optical beams with an expanded exit pupil in one or two dimensions.

p-0008According further to the first aspect of the invention, the apparatus may be configured using at least one condition of: a) two areas, each occupied by one of the two diffractive elements, are symmetric relative to the line which separates the first and the second parts, and b) two further areas, each occupied by one of the two further diffractive elements, are symmetric relative to the line which separates the first and the second parts.

p-0009According further to the first aspect of the invention, the two diffractive elements may have an asymmetric groove shape such that the input optical beam diffracted by each of the two diffractive elements may be substantially coupled only to a part, out of the first and the second parts, in which the each of the two diffractive elements may be disposed.

p-0010Still further according to the first aspect of the invention, the two diffractive elements may have an asymmetric groove shape and may be slanted gratings with a slanting angle of more than 20 degrees.

p-0011According further to the first aspect of the invention, the two diffractive elements may be asymmetric such that their groove shapes are mirror images of each other with respect to the line which separates the first and the second parts.

p-0012According still further to the first aspect of the invention, the two diffractive elements and the two further diffractive elements may be disposed on one surface, the first surface or the second surface of the first and second parts of the split substrate.

p-0013According further still to the first aspect of the invention, an absorbing material may be deposited on at least one of: a) an end of at least one of the first and the second parts in an area of their physical separation along the line, and b) a surface of the split substrate opposite to the surface with disposed the two diffractive elements.

p-0014According yet further still to the first aspect of the invention, the grooves of the two diffractive elements may be symmetric relative to the line which separates the first and the second parts.

p-0015Yet still further according to the first aspect of the invention, each part, the first and the second part of the split substrate, may comprise an intermediate diffractive element such that the at least part of the optical beam diffracted in the first or the second diffractive element is first coupled to the intermediate diffractive element, which then couples, using a further diffraction in the intermediate diffractive element, the at least part of the diffracted optical beam to one of the two further diffractive elements disposed on the each part, to provide a two-dimensional exit pupil expansion of the input optical beam by the each part. Further, the apparatus may be configured that the intermediate diffractive element has an odd number of first order diffractions or an even number of further first order reflections. Still further, each of the two diffractive elements comprises periodic lines with a period d and the intermediate diffractive element comprises further periodic lines with a period d′ which may be equal to:

p-0016<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mrow><mrow><msup><mi>d</mi><mi>′</mi></msup><mo>=</mo><mfrac><mi>d</mi><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mfrac><mi>d</mi><mi>λ</mi></mfrac><mo>⁢</mo><mi>sin</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo>⁢</mo><mn>2</mn><mo>⁢</mo><mi>cos</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>ρ</mi></mrow></mfrac></mrow><mo>,</mo></mrow></math></maths>

p-0017wherein α is an angle between a normal to the first part or the second part and a normal to a plane comprising the first part and the second part when the first part and the second part are rotated in the predetermined angle range to lie both in said plane, ρ is an angle between the periodic lines and the further periodic lines, and λ is a wavelength of the input optical beam.

p-0018According to a second aspect of the invention, a method, comprises: receiving an input optical beam by two diffractive elements disposed on a first or a second surface of a split substrate made of optical material, the split substrate comprising a first part and a second part which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range, and wherein one of the two diffractive elements is disposed on the first part and another of the two diffractive elements is disposed on the second part, respectively, and the two diffractive elements are substantially next to each other and adjacent to the line which separates the first and the second parts, and two areas, each occupied by one of the two diffractive elements, are symmetric relative to the line which separates the first and the second parts; diffracting at least part of the input optical beam in the two diffractive elements to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces; coupling at least part of the diffracted optical beams in each of the first part and the second part out of the split substrate by diffraction in each of two further diffractive elements to provide substantially identical two output expanded optical beams with expanded exit pupil in one or two dimensions, wherein the two further diffractive elements are disposed on the first or the second surface, wherein one of the two further diffractive elements is disposed on the first part and another of the two further diffractive elements is disposed on the second part, respectively.

p-0019According further to the second aspect of the invention, the two diffractive elements may have an asymmetric groove shape such that the input optical beam diffracted by each of the two diffractive elements is substantially coupled only to a part, out of the first and the second parts, in which the each of the two diffractive elements is disposed.

p-0020Further according to the second aspect of the invention, the two diffractive elements may be asymmetric such that their groove shapes are mirror images of each other with respect to the line which separates the first and the second parts.

p-0021Still further according to the second aspect of the invention, an absorbing material may be deposited on at least one of: a) an end of at least one of the first and the second parts in an area of their physical separation along the line, and b) a surface of the split substrate opposite to the surface with disposed the two diffractive elements.

p-0022According to a third aspect of the invention, an electronic device, comprises: <ul><li id="ul0001-0001" num="0000"><ul><li id="ul0002-0001" num="0022">a data processing unit;</li><li id="ul0002-0002" num="0023">an optical engine operatively connected to the data processing unit for receiving image data from the data processing unit;</li><li id="ul0002-0003" num="0024">a display device operatively connected to the optical engine for forming an image based on the image data; and</li><li id="ul0002-0004" num="0025">an exit pupil expander comprising:</li><li id="ul0002-0005" num="0026">a split substrate of optical material having a first surface and a second surface, the split substrate comprising a first part and a second part which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range;</li><li id="ul0002-0006" num="0027">two diffractive elements disposed on the first or the second surface and configured to receive an input optical beam, wherein one of the two diffractive elements is disposed on the first part and another of the two diffractive elements is disposed on the second part, respectively, and the two diffractive elements are substantially next to each other and adjacent to the line which separates the first and the second parts; and</li><li id="ul0002-0007" num="0028">two further diffractive elements disposed on the first or the second surface, wherein one of the two further diffractive elements is disposed on the first part and another of the two further diffractive elements is disposed on the second part, respectively, wherein</li><li id="ul0002-0008" num="0029">at least part of the input optical beam is diffracted in the two diffractive elements to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces, and</li><li id="ul0002-0009" num="0030">at least part of the diffracted optical beams in each of the first part and the second part is further coupled out of the split substrate by diffraction in each of the two further diffractive elements to provide substantially identical two output optical beams with an expanded exit pupil in one or two dimensions.</li></ul></li></ul>

p-0023Further according to the third aspect of the invention, the electronic device may further comprise: an EPE angle adjustment block configured to rotate the first part and the second part relative to each other around the line.

p-0024Still further according to the third aspect of the invention, the electronic device may be a digital camera, a computer game device, a wireless device, a portable device or a mobile terminal.

p-0025According further to the third aspect of the invention, the electronic device may be configured using at least one condition of: a) two areas, each occupied by, one of the two diffractive elements, are symmetric relative to the line which separates the first and the second parts, and b) two further areas, each occupied by one of the two further diffractive elements, are symmetric relative to the line which separates the first and the second parts.

p-0026According still further to the third aspect of the invention, the two diffractive elements may have an asymmetric groove shape such that the input optical beam diffracted by each of the two diffractive elements is substantially coupled only to a part, out of the first and the second parts, in which the each of the two diffractive elements is disposed.

p-0027According yet further still to the third aspect of the invention, the two diffractive elements may be asymmetric such that their groove shapes are mirror images of each other with respect to the line which separates the first and the second parts.

p-0028According further still to the third aspect of the invention, the two diffractive elements and the two further diffractive elements may be disposed on one surface, the first surface or the second surface of the first and second parts of the split substrate.

p-0029Yet still further according to the third aspect of the invention, an absorbing material may be deposited on at least one of: a) an end of at least one of the first and the second parts in an area of their physical separation along the line, and b) a surface of the split substrate opposite to the surface with disposed the two diffractive elements.

p-0030According to a fourth aspect of the invention, a goggle, comprises: <ul><li id="ul0003-0001" num="0000"><ul><li id="ul0004-0001" num="0039">a wearable housing; and</li><li id="ul0004-0002" num="0040">an exit pupil expander, operatively attached to the wearable housing, the exit pupil expander comprising:</li></ul></li></ul>

p-0031a split substrate of optical material having a first surface and a second surface, the split substrate comprising a first part and a second part which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range; two diffractive elements disposed on the first or the second surface and configured to receive an input optical beam, wherein one of the two diffractive elements is disposed on the first part and another of the two diffractive elements is disposed on the second part, respectively, and the two diffractive elements are substantially next to each other and adjacent to the line which separates the first and the second parts; and two further diffractive elements disposed on the first or the second surface, wherein one of the two further diffractive elements is disposed on the first part and another of the two further diffractive elements is disposed on the second part, respectively, wherein at least part of the input optical beam is diffracted in the two diffractive elements to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces, and at least part of the diffracted optical beams in each of the first part and the second part is further coupled out of the split substrate by diffraction in each of the two further diffractive elements to provide substantially identical two output optical beams with an expanded exit pupil in one or two dimensions, such that each of the two output optical beams is provided to only one part, left or right, of the goggles.

p-0032According further to the fourth aspect of the invention, the goggle may further comprise: an EPE angle adjustment block, configured to provide an adjustment signal to the exit pupil expander for rotating the first and second parts relative to each other in the predetermined angle range.

p-0033According to a fifth aspect of the invention, an apparatus, comprises:

p-0034two means for diffraction, <ul><li id="ul0005-0001" num="0000"><ul><li id="ul0006-0001" num="0045">for receiving an input optical beam, wherein the two means for diffraction are disposed on a first or a second surface of a split substrate made of optical material, the split substrate comprising a first part and a second part which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range, and wherein one of the two means for diffraction is disposed on the first part and another of the two means for diffraction is disposed on the second part, respectively, and the two means for diffraction are substantially next to each other and adjacent to the line which separates the first and the second parts, and two areas, each occupied by one of the two means for diffraction, are symmetric relative to the line which separates the first and the second parts, and</li><li id="ul0006-0002" num="0046">for diffracting at least part of the input optical beam in the two means for diffraction to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces; and</li></ul></li></ul>

p-0035two further means for diffraction, for coupling at least part of the diffracted optical beams in each of the first part and the second part out of the split substrate by diffraction in each of two further means for diffraction to provide substantially identical two output expanded optical beams with expanded exit pupil in one or two dimensions,

p-0036wherein the two further means for diffraction are disposed on the first or the second surface, wherein one of the two further means for diffraction is disposed on the first part and another of the two further diffractive elements is disposed on the second part, respectively.

p-0037According further to the fifth aspect of the invention, the two means for diffraction may be two in-coupling diffraction gratings and the two further means for diffraction may be two out-coupling diffraction gratings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the present invention, reference is made to the following detailed description taken in conjunction with the following drawings, in which:

<figref idrefs="DRAWINGS">FIGS. 1</figref><i>a </i>and <b>1</b><i>b </i>are schematic representations of a virtual reality display with a diffractive exit pupil expander as shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>a </i>and of an out-coupling grating of the diffractive exit pupil expander, shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>b; </i>

<figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b </i>are schematic representations (cross-sectional and top views shown in <figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b</i>, respectively) of a split one-dimensional diffractive exit pupil expander, according to an embodiment of the present invention;

<figref idrefs="DRAWINGS">FIGS. 3</figref><i>a </i>and <b>3</b><i>b </i>are schematic representations of a split in-coupling grating using an exit pupil expander, according to embodiments of the present invention;

<figref idrefs="DRAWINGS">FIGS. 4</figref><i>a </i>and <b>4</b><i>b </i>are schematic representations of one part (out of two) of a split two-dimensional diffractive exit pupil expander, wherein an intermediate diffractive element (grating) has an odd number of first order diffractions (shown in <figref idrefs="DRAWINGS">FIG. 4</figref><i>a</i>) or an even number of further first order reflections (shown in <figref idrefs="DRAWINGS">FIG. 4</figref><i>b</i>), according to an embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 5</figref> is a schematic representation of a flat diffractive exit pupil expander in a “sunglass like” frame, according to an embodiment of the present invention; and

<figref idrefs="DRAWINGS">FIG. 6</figref> is a schematic representation of an electronic device, having an exit pupil expander, according to an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

p-0045A new method and apparatus are presented for using a split exit pupil expander to provide (instead of a flat exit pupil expander) a general diffractive optics method that uses a plurality of diffractive elements for expanding the exit pupil of a display of an electronic device for viewing. The embodiments of the present invention can be applied to a broad optical spectral range of optical beams but most importantly to a visible part of the optical spectrum where the optical beams are called light beams.

p-0046According to embodiments of the present invention, the optical device (e.g., the optical device is a part of a virtual reality display) can comprise a split substrate made of optical material having a first surface and an opposing second surface, wherein the split substrate comprising a first part and a second part (or left and right part) which are physically separated and configured to rotate relative to each other around a line which separates the first and the second parts in a predetermined angle range.

p-0047Moreover, two diffractive elements (or in-coupling diffraction gratings) can be disposed on the first or the second surface and configured to receive an input optical beam, wherein one of these two diffractive elements is disposed on the first part and another of the two diffractive elements is disposed on the second part, respectively, and said two diffractive elements are substantially next to each other and adjacent to the line which separates the first and the second parts, and two areas, each occupied by one of said two diffractive elements, can be symmetric relative to the line which separates the first and the second parts. Thus, at least part of the input optical beam is diffracted in the two diffractive elements to provide two diffracted optical beams substantially equally divided between the first part and the second part, respectively, substantially within the first and second surfaces due to a total internal reflection. Then the two parts can expand the exit pupil of the input optical beam independently in one or two dimensions to provide substantially identical two output optical beams.

p-0048In case of a simple one-dimensional exit pupil expansion, two further diffractive elements (or out-coupling diffraction gratings) can be disposed on the first or the second surface (e.g., the two further diffractive elements may have parallel periodic lines), wherein one of the two further diffractive elements is disposed on the first part and another of said two diffractive elements is disposed on the second part, respectively, and two further areas, each occupied by one of said two further diffractive elements, can be symmetric relative to the line which separates the first and the second parts, thus at least part of the diffracted optical beams in each of the first part and the second part of the split substrate is further coupled out of the split substrate by diffraction (as known in the art) in each of the two further diffractive elements, thus providing substantially identical two output optical beams. It is noted that the two diffractive elements and the two further diffractive elements can be disposed on one surface or on different surfaces of said split substrate.

p-0049In case of a two-dimensional exit pupil expansion, each of the first and the second parts of the split substrate can comprise an intermediate diffractive element such that the at least part of the optical beam diffracted in the first or the second diffractive element is first coupled to the intermediate diffractive element, which then couples, using a further diffraction in the intermediate diffractive element, the at least part of the diffracted optical beam to one of the two further diffractive elements disposed on each part, thus providing the two-dimensional exit pupil expansion of the input optical beam by the each of the first and the second parts. The intermediate diffractive element can have an odd number of first order diffractions or an even number of further first order reflections as known in the art and, e.g., described by T. Levola in “Diffractive Optics for Virtual Reality Displays”, SID Eurodisplay 05, Edinburg (2005), SID 02 Digest, Paper 22.1.

p-0050According to embodiments of the present invention, the two diffractive elements (or the in-coupling diffraction gratings) can be implemented using a variety of different types of diffraction gratings, e.g., planar diffraction gratings manufactured using lithographic methods or classically ruled (having different groove angles and profiles, such as binary, triangular, sinusoidal, etc.). The two diffractive elements (i.e., their grooves) can be symmetric or asymmetric relative to the line which separates the first and the second parts. The term “asymmetric” in regard to the grooves of the two in-coupling gratings can have two aspects: a) when periodic lines (or grooves) of the two gratings are not parallel, and b) when grooves of the two gratings have different slanted angles. Therefore, one possibility is to have non-parallel asymmetric periodic lines in the two in-coupling diffraction gratings, thus re-directing only wanted components in each part of the substrate to the corresponding out-coupling gratings. Another solution (which can be combined with periodic line asymmetry) is to use slanted gratings (e.g., using a slanting angle of at least more than 20 degrees and optimally between 35 and 50 degrees) for increasing the coupling efficiency and reducing an “optical crosstalk” between the first and the second parts (or left and right parts) of the split substrate. In other words, the asymmetric gratings (used as the two diffractive elements) can provide that the input optical beam diffracted by each of the two diffractive elements is substantially coupled only in a desired direction to the part in which said each of the two diffractive elements is disposed.

p-0051Furthermore, according to an embodiment of the present invention, the two slanted gratings are asymmetric such that their slanting angles are equal but have opposite signs relative to the optical axis of the system creating the input optical beam, i.e., the groove shapes are mirror images of each other with respect to the line which separates the first and the second parts. This minimizes the optical crosstalk between the two parts of the split substrate (i.e., to improve the image contrast). Moreover, an absorbing material can be deposited on the first and/or the second part in an area of their physical separation along the line which separates the first and the second parts. The contrast can be further improved by providing an absorbing material on the opposite surface of the split substrate (i.e., opposite to the surface with the disposed input diffractive elements). The width of this absorbing material should be optimized (e.g., to be approximately the same as the thickness of the substrate) in order to absorb only optical beams propagating in unwanted directions.

p-0052<figref idrefs="DRAWINGS">FIG. 1</figref><i>a </i>and <figref idrefs="DRAWINGS">FIG. 1</figref><i>b </i>show examples among others of schematic representations of: a virtual reality display with a diffractive exit pupil expander (EPE) as shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>a </i>providing two substantially identical images for the right and left eyes using an out-coupling grating, shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>b</i>. The light is coupled out from the out-coupling grating. The amount of out-coupling at each time the beam meets the grating depends on the grating properties. The system can be designed so that at least for one wavelength and incoming angle the output is uniform, i.e. r₁=r₂= . . . , as shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>b</i>, wherein r₁, r₂, . . . and t₁, t₂, . . . are reflected and transmitted optical beams out of the EPE, respectively, and I<b>1</b>, I<b>2</b> . . . are reflected optical beams inside the EPE by the total internal reflection. The example of the virtual reality display of <figref idrefs="DRAWINGS">FIG. 1</figref><i>a </i>with the out-coupling grating shown in <figref idrefs="DRAWINGS">FIG. 1</figref><i>b</i>, can be used for applying embodiments of the present invention. <figref idrefs="DRAWINGS">FIGS. 2-6</figref> demonstrate different embodiments of the present invention.

p-0053<figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b </i>show examples among others of schematic representations (cross-sectional and top views shown in <figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>and <b>2</b><i>b</i>, respectively) of a split one-dimensional diffractive exit pupil expander (EPE) <b>10</b>, according to an embodiment of the present invention. The EPE <b>10</b> comprises a split substrate comprising two adjacent parts <b>12</b><i>a </i>and <b>12</b><i>b </i>that are physically separated on a line <b>18</b> two parts. These two parts <b>12</b><i>a </i>and <b>12</b><i>b </i>are configured to rotate in a direction <b>15</b> relative to each other around the line <b>18</b> in a predetermined angle range which provides the flexibility in the virtual reality display design (angle α is an angle between the adjacent part <b>12</b><i>a </i>or <b>12</b><i>b </i>and a plane <b>13</b> which comprises the two parts <b>12</b><i>a </i>and <b>12</b><i>b </i>in a flat position, which is equivalent to an angle between a normal to the adjacent part <b>12</b><i>a </i>or <b>12</b><i>b </i>and a normal to the horizontal plane <b>13</b>). The part <b>12</b><i>a </i>comprises the in-coupling grating <b>14</b><i>a </i>and an out-coupling grating <b>16</b><i>a </i>and the part <b>12</b><i>b </i>comprises the in-coupling grating <b>14</b><i>b </i>and an out-coupling grating <b>16</b><i>b</i>, respectively. The adjacent gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>can be identical (or symmetric) or asymmetric as further shown in <figref idrefs="DRAWINGS">FIG. 3</figref>. It is noted that the in-coupling gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>can generally be means for diffraction or a structural equivalence (or an equivalent structure) thereof. Similarly, the out-coupling gratings <b>16</b><i>a </i>and <b>16</b><i>b </i>can generally be further means for diffraction or a structural equivalence (or equivalent structure) thereof.

p-0054<figref idrefs="DRAWINGS">FIG. 3</figref><i>a </i>is one example among others of a schematic representation of a split exit pupil expander <b>10</b> using slanted asymmetric in-coupling gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>(diffractions grooves of gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>face different direction relative to the optical axis of the system creating the input optical beam), according to an embodiment of the present invention. According to a further embodiment, the ends of the diffraction gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>can be coated with absorbing materials (e.g., coatings) <b>20</b><i>a </i>and <b>20</b><i>b </i>along the line <b>18</b> to further isolate (optically) the two parts <b>12</b><i>a </i>and <b>12</b><i>b. </i>

p-0055The optical contrast can be further improved by providing absorbing materials (e.g., an absorbing coating) <b>17</b><i>a </i>and <b>17</b><i>b </i>on the surfaces of the split substrates <b>12</b><i>a </i>and <b>12</b><i>b </i>opposite to the substrate surface with the disposed input diffraction gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>in a vicinity of the line <b>18</b> (as shown in <figref idrefs="DRAWINGS">FIG. 3</figref><i>a</i>). If the width of the absorbing area is optimized to be small enough compared to the total width of the gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>as shown in <figref idrefs="DRAWINGS">FIG. 3</figref><i>a</i>, only the unwanted optical beams will be absorbed. These unwanted beams are the optical beams which are transmitted by the gratings <b>14</b><i>a </i>and <b>14</b><i>b </i>without diffracting and those diffracted beams that propagate in unwanted directions. The absorbing materials <b>17</b><i>a </i>and <b>17</b><i>b </i>can be used in addition or instead of absorbing material <b>20</b><i>a </i>and <b>20</b><i>b. </i>

p-0056<figref idrefs="DRAWINGS">FIG. 3</figref><i>b </i>is another example among others of a schematic representation of a split exit pupil expander <b>10</b> using slanted asymmetric in-coupling gratings <b>14</b><i>a </i>and <b>14</b><i>b</i>, according to an embodiment of the present invention. The configuration is similar to the expander <b>10</b> shown in <figref idrefs="DRAWINGS">FIG. 3</figref><i>a </i>but with the input optical beam coming from the side of the observer, which makes the system slightly more compact.

p-0057<figref idrefs="DRAWINGS">FIGS. 4</figref><i>a </i>and <b>4</b><i>b </i>show further examples among others of schematic representations of one part (<b>12</b><i>a </i>or <b>12</b><i>b</i>) of split two-dimensional diffractive exit pupil expanders <b>10</b><i>a </i>and <b>10</b><i>b</i>, respectively, according to an embodiment of the present invention. An intermediate diffractive element (a diffraction grating) <b>24</b> or <b>26</b> has odd number of first order diffractions (shown in <figref idrefs="DRAWINGS">FIG. 4</figref><i>a</i>) or even number of further first order reflections (shown in <figref idrefs="DRAWINGS">FIG. 4</figref><i>b</i>) as described by T. Levola in “Diffractive Optics for Virtual Reality Displays”, SID Eurodisplay 05, Edinburg (2005), SID 02 Digest, Paper 22.1. The angle ρ is a rotation angle between the periodic lines of the intermediate diffraction grating <b>24</b> or <b>26</b> and the in-coupling split grating <b>14</b><i>a </i>or <b>14</b><i>b. </i>

p-0058According to a further embodiment of the present invention, the relationship between a period d of periodic lines of the in-coupling grating <b>14</b><i>a </i>or <b>14</b><i>b </i>and a period d′ of further periodic lines of the intermediate diffraction grating <b>16</b><i>a </i>or <b>16</b><i>b </i>can be expressed as follows:

p-0059<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><mrow><msup><mi>d</mi><mi>′</mi></msup><mo>=</mo><mfrac><mi>d</mi><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mrow><mfrac><mi>d</mi><mi>λ</mi></mfrac><mo>⁢</mo><mi>sin</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>α</mi></mrow></mrow><mo>)</mo></mrow><mo>⁢</mo><mn>2</mn><mo>⁢</mo><mi>cos</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>ρ</mi></mrow></mfrac></mrow><mo>,</mo></mrow></math></maths>

p-0060wherein α is an angle between the adjacent parts <b>12</b><i>a </i>or <b>12</b><i>b </i>and a plane <b>13</b> as shown in <figref idrefs="DRAWINGS">FIG. 2</figref><i>a</i>, ρ is an angle between the periodic lines of the in-coupling grating <b>14</b><i>a </i>or <b>14</b><i>b </i>and the further periodic lines of the intermediate diffraction grating <b>16</b><i>a </i>or <b>16</b><i>b </i>as shown in <figref idrefs="DRAWINGS">FIGS. 4</figref><i>a </i>and <b>4</b><i>b</i>, and λ is a wavelength of the input optical beam.

p-0061<figref idrefs="DRAWINGS">FIG. 5</figref> show further examples among others of schematic representations of a split diffractive exit pupil expander <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b </i>in a goggle <b>11</b> comprising a wearable housing <b>11</b><i>a</i>, according to an embodiment of the present invention. The exit pupil expander <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b </i>is operatively attached to the wearable housing <b>11</b><i>a </i>such that the split parts (the first and the second parts) of the split EPE <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b </i>can be rotated in the predetermined angle range, according to embodiments of the present invention. The wearable housing <b>11</b><i>a </i>can be a glass housing, such as a sunglass frame, or a spectacle frame or alike. <figref idrefs="DRAWINGS">FIG. 5</figref> also shows an EPE angle adjustment block <b>30</b> which can be a part of the frame <b>11</b> and which can be used to provide an adjustment signal <b>30</b><i>a </i>for rotating the split parts of the split EPE <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b </i>in the predetermined angle range to provide the most comfortable and efficient position for viewing.

p-0062<figref idrefs="DRAWINGS">FIG. 6</figref> shows an example of a schematic representation of an electronic device, having the exit pupil expander (EPE) <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b</i>, according to an embodiment of the present invention.

p-0063The exit pupil expander (EPE) <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b </i>can be used in an electronic (portable) device <b>100</b>, such as a mobile phone, personal digital assistant (PDA), communicator, portable Internet appliance, hand-hand computer, digital video and still camera, wearable computer, computer game device, specialized bring-to-the-eye product for viewing and other portable electronic devices. As shown in <figref idrefs="DRAWINGS">FIG. 6</figref>, the portable device <b>100</b> has a housing <b>210</b> to house a communication unit <b>212</b> for receiving and transmitting information from and to an external device (not shown). The portable device <b>100</b> also has a controlling and processing unit <b>214</b> for handling the received and transmitted information, and a virtual display system <b>230</b> for viewing. The virtual display system <b>230</b> includes a micro-display or an image source <b>192</b> and an optical engine <b>190</b>. The controlling and processing unit <b>214</b> is operatively connected to the optical engine <b>190</b> to provide image data to the image source <b>192</b> to display an image thereon. The EPE <b>10</b>, <b>10</b><i>a </i>or <b>10</b><i>b</i>, according to the present invention, can be optically linked to an optical engine <b>190</b>.

p-0064Furthermore, the image source <b>192</b>, as depicted in <figref idrefs="DRAWINGS">FIG. 6</figref>, can be a sequential color LCOS (Liquid Crystal On Silicon) device, an OLED (Organic Light Emitting Diode) array, an MEMS (MicroElectro Mechanical System) device or any other suitable micro-display device operating in transmission, reflection or emission.

p-0065Moreover, the electronic device <b>100</b> can be a portable device, such as a mobile phone, personal digital assistant (PDA), communicator, portable Internet appliance, hand-held computer, digital video and still camera, wearable computer, computer game device, specialized bring-to-the-eye product for viewing and other portable electronic devices. However, the exit pupil expander, according to the present invention, can also be used in a non-portable device, such as a gaming device, vending machine, band-o-matic, and home appliances, such as an oven, microwave oven and other appliances and other non-portable devices.

p-0066It is noted that various embodiments of the present invention recited herein can be used separately, combined or selectively combined for specific applications.

p-0067It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the present invention, and the appended claims are intended to cover such modifications and arrangements.